Generally speaking, conventional die casting machines and injection molding machines include a stationary platen that supports a stationary mold, a movable platen that supports a movable mold, and a supporting stage. The two ends of each of a plurality of (normally four) tie bars that extend through the movable platen are connected to the stationary platen and the supporting stage respectively. In the conventional machines, the mold closing process and the mold clamping process are performed by moving the movable platen toward the stationary platen side along the tie bars, using a mold clamping mechanism provided between the supporting stage and the movable platen.
With regard to the mold clamping mechanism, a toggle mold clamping mechanism has been popularly used in large die casting machines and injection molding machines, because it provides a strong mold clamping force that can withstand a large internal mold pressure. However, the toggle mold clamping mechanism has the problem where not only the parts (for example, a link) that structure the toggle mechanism are complicated and large-sized, but also the mechanism requires a large installation space because a long-stroke mold clamping cylinder needs to be used.
To cope with this problem, a mold clamping device using a tie-bar direct pressure method by which a mold clamping force is generated via the tie bars has been developed and put into use. Some mold clamping devices of this type have a structure in which one end of each of tie bars is directly connected to a ram (a piston) inside a hydraulic cylinder (a mold clamping cylinder) provided on the back of a stationary die plate (a stationary platen) (See Japanese Patent Application Publication No. 2001-1381). These days, however, attention is focused on a mold clamping device in which the tie bars can be removed, for the purpose of also improving the efficiency in the mold changing process.
A mold clamping device in which the tie bars can be removed according to a conventional technique is disclosed in Japanese Patent Application Publication No. H8-72113, for example. This device includes a stationary platen that supports a stationary mold, a movable platen (a movable die plate) that supports a movable mold, a plurality of tie bars with ends that are detachably connected to the stationary platen and other ends that extend through the movable platen, mold opening and closing means that moves the movable platen toward and away from the stationary platen so that the movable mold opens from and closes on the stationary mold, tie bar connecting means that detachably connects each of the plurality of tie bars to the movable platen by having a split nut engaged with an engagement portion formed in each of the tie bars, and a mold clamping cylinder that is provided around a tie bar insertion through hole in the movable platen and propels the movable platen toward the stationary platen side, using the split nut included in the tie bar connecting means as a reaction point, so as to generate a mold clamping force. According to the mold clamping device disclosed in the Japanese Patent Application Publication No. H8-72113, because the mold clamping cylinder is incorporated into the stationary platen, the size of the whole device is compact, and also, it is possible to remove the tie bars from the stationary platen by operating the mold opening and closing means after the fixation of the tie bars to the stationary platen is released.
According to the mold clamping device disclosed in the Japanese Patent Application Publication No. H8-72113, however, the tie bars are connected to the movable platen by the tie bar connecting means, after the movable platen is moved by the mold opening and closing means until the movable mold is in contact with the stationary mold. Thus, in the case where the thickness of the mold changes due to a change in the mold temperature, a problematic situation is often experienced where the split nut included in the tie bar connecting means cannot be engaged with the engagement portion of the tie bar. In particular, when the mold clamping device is applied to a die casting apparatus, which has a large fluctuation of mold temperature, this problematic situation is notably present, and the problem arises where the stability of the molding process is significantly degraded.
In order to solve this problem, the stroke of the mold opening and closing means should be adjusted so that the movable platen is stopped at a position that is immediately before the movable mold comes in contact with the stationary mold and by which the split nut included in the tie bar connecting means can be securely engaged with the engagement portion of the tie bar. According to the mold clamping device disclosed in the Japanese Patent Application Publication No. H8-72113, however, because the piston stroke is regular, if the piston stroke before the mold contact is made shorter in order to put emphasis on the molding cycles (i.e. productivity), the piston stroke at the time when the mold is released (i.e. the mold release stroke) becomes insufficient, and an error will occur in the mold release process. Consequently, there is a constraint that it is necessary to make the piston stroke before the mold contact have a certain length.